As an example of conventional technology relating to the present invention, there is a computer system described in Japanese Patent Application Laid-Open (Kokai) Publication No. 2008-146574. This patent literature discloses that when resident software of a host computer cancels the correspondence relationship between data in a file system and areas (pages) of virtual volumes, specified data is written to an area of a virtual volume and the area of the virtual volume to which the specified data is written is then returned to a pool by a storage control device.
Japanese Patent Application Laid-Open (Kokai) Publication No. 2010-0033374 which is another conventional technology discloses that when deleting a file or a snapshot, a file management device judges whether or not data to be deleted is shared with another snapshot; and if the data to be deleted is not shared, zero data is written to an area of the data to be deleted.